ConcurrentUpdateSolrServer hang in blockUntilFinished.

Details

Description

Not sure what is causing this, but SOLR-6136 may have taken us a step back here. I see this problem occasionally pop up in ChaosMonkeyNothingIsSafeTest now - test fails because of a thread leak, thread leak is due to a ConcurrentUpdateSolrServer hang in blockUntilFinished. Only started popping up recently.